I need to change the CKSEL fuses on one of them to 0000 so I supply it with an external clock (which I will provide from the other '328).

Was gonna do it with AVR ISP MKii, but AVR Studio 5 doesn't seem to recognize it as being plugged in, I am guessing as a result of changing from the AVR Studio default Jungo driver, to the Arduino provided driver for the MKii.

Note: 1. The default value of SUT1...0 results in maximum start-up time for the default clock source.See Table 9-12 on page 34 for details.2. The default setting of CKSEL3...0 results in internal RC Oscillator @ 8MHz. See Table 9-11 onpage 34 for details.3. The CKOUT Fuse allows the system clock to be output on PORTB0. See "Clock Output Buffer"on page 36 for details.4. See "System Clock Prescaler" on page 36 for details.

So have bit 7 = 0 means Divide by 8 is enabled? I still want to run at 16 MHz. The clock will be coming from Xtal2 on the 2nd 328 in the dual-328 system I am proofing out, and go into Xtal1.PB0 will not be changed, and PB7 should be free'd up as well. I suppose that could be added to pins_arduinio.h as well to have another IO pin.

This part here changes the CKSEL bits? lfuse:w:0xe0:mWhy does the upper bit of 0xff get changed as well?

That's it. Zero = programmed, so the default lfuse value of 0x62 enables the clock divider and results in a 1MHz clock. Interestingly the clock prescaler can be overridden in software by writing to CLKPR. Handy for testing code on a 5V 16MHz board before building a project based on a lower-frequency board. It can also be used for power saving without the bother of entering and exiting sleep mode, although I haven't tried this myself.

One protocol I have been very happy with is EasyTransfer by Bill Porter. Available as a I2C as well as Serial library. Makes communications between two Arduinos a snap. Used it for 2 328's, a 328 and a 1284p, etc. Highly recommended.

@tim7, You are right - I was flipping the wrong bit of the nibble. 0xe0 it is.

@Constantin, Hadn't got that far yet. Still working on getting 2 uCs running of 1 crystal so I can some boards ordered. I think I have enough jumpers/headers on the board so any of those kinds of comm's can be set up.See the schematic in Reply #9 of this thread. Have to tweak the crystal connections a little - xtal2 of chip1 will feed xtal1 of chip2. chip2 will have fuses set to allow external clock.Think I'll do the connections with solder jumper pads so the two can be isolated if needed.

No worries. Just thought I'd chime in re: using the I2C bus for the communications. Pretty easy to implement and requires few traces and components on the board. Maybe give a choice using a shunt jumper or a cutable pad for SDA and SCL?